DETROIT (Reuters) – Bondholders holding some $27 billion of General Motors Corp (GM.N) debt on Sunday sent a letter to the White House task force overseeing the bailout of the auto industry, to register concern with the lack of progress in GM's debt restructuring talks.

In an open letter to U.S. Treasury Secretary Timothy Geithner and other members of the U.S. autos task force, representatives of GM bondholders said they had not had a response from U.S. officials or GM since proposing terms for a debt swap earlier this month.

"Keeping lines of communications open is the only way we all can meet the March 31 deadline for a debt-to-equity exchange," the letter said. "We are disappointed that we have had no response to our proposal from either GM or the auto task force."

The letter on behalf of GM bondholders was signed by advisers to an ad hoc committee negotiating a debt restructuring with the automaker prompted by the terms of its $13.4 billion bailout.